Generally, ion-exchange chromatography employs the use of columns. These columns are packed with a resin, often in the form of granules having sorptively active surfaces or surfaces which have been coated with a substance which is sorptively active.
It is well recognized in the art that excellent chromatographic supports consist of a plurality of discrete particles of regular shape, preferably spheres, having surfaces with a large number of superficial shallow pores. In order that columns will give reproducible chromatographic results, support particles ideally should be regular in their surfaces and their surface characteristics easily reproducible.
Materials for performing liquid chromatographic analyses are known where only the thin outer surfaces of the chromatographic support materials are available for actively exchanging ions with liquid media. For example, Small et al., in U.S. Pat. No. 4,101,460 (1978) describes the preparation and use of an ion-exchange composition comprising Component A, an insoluble synthetic resin substrate having ion-exchanging sites, at least on its available surface, and having Component B, a finely divided insoluble material, irreversibly attached thereto.
In the above prior art electrostatic methods, the resin support particles are typically formed by a suspension polymerization process. A dispersant material is frequently used in polymerization of the resin support particles to maintain separate particles in the reaction solution as they are produced, preventing the desired size particles from sticking to each other and forming a larger agglomerate particle. The resin support particles are then lightly sulfonated by exposing them at room temperature to concentrated sulfuric acid for a few minutes. This creates a very thin layer of sulfonate sites on the surface of the resin support particle and allows for electrostatic attachment of fine resin layering particles such as aminated latex beads.
Thus, in the above prior art electrostatic methods it is necessary to functionalize the fine layering particles by creating a positive or negative charge, at least at the surfaces of those particles, for the electrostatic attachment. This has been done by aminating or sulfonating latex-derived particles. If the latex is aminated, the resin support particles are sulfonated. In the case where the latex is sulfonated, the resin support particles are aminated. The latex and support particles are then brought into contact with each other, resulting in a monobead coat of latex particles electrostatically attached to the surface of the resin support particles. This produces a pellicular anion-exchange or pellicular cation-exchange resin bead.
Barretto et al., U.S. Pat. No. 5,532,279 (“the '279 patent”) describe at column 1-3 various prior art processes for making agglomerates of such ion-exchange Component B particles layered electrostatically onto Component A particles. Further, it describes an ion-exchange composition comprising synthetic resin support particles, dispersant capable of suspending the support particles in an aqueous medium to inhibit or prevent agglomeration, and fine synthetic resin layering particles. The complex can be formed by contacting a suitable dispersant with monomer in an aqueous solution in which the monomer is insoluble. Under suitable conditions for suspension polymerization, the monomer will polymerize to form resin support particles having dispersant irreversibly attached to those particles. The dispersant is irreversibly attached to the synthetic resin support particles either by covalent bonding or permanent physical entanglement. The dispersant is also attached to the fine layering particles, either by covalent bonding or by electrostatic forces. The result is formation of a support particle-dispersant-layering particle complex.
The '279 patent discloses that the ion-exchange composition can be made by forming a synthetic resin support particle-dispersant complex in which the dispersant is irreversibly attached to the support particles. The complex can then be mixed in aqueous slurry form with fine resin layering particles under conditions suitable to form a resin support particle-dispersant-resin layering particle complex in which the resin layering particles are irreversibly attached to the dispersant. In effect, the dispersant forms an irreversible bridge between the support and layering particles. The dispersant is irreversibly attached to the resin support particles. The dispersant contains ionic or potentially reactive functional groups that can be exploited through further chemistry after polymerization of the support particles. Ion-exchange compositions of the type described in the '279 patent are sold by Thermo Fisher Scientific under the trade names OmniPac™ including the OmniPac™ PAX-100, the OmniPac™ PAX-500 the OmniPac™ PCX-100 and the OmniPac™ PCX-500 columns.